Re: [PATCH] x86/MTRR: hook mtrr_bp_restore() back up

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Mar 27, 2025 at 02:28:42PM +0100, Jan Beulich wrote:
> On 27.03.2025 13:48, Roger Pau Monné wrote:
> > On Thu, Mar 27, 2025 at 01:30:44PM +0100, Jan Beulich wrote:
> >> On 27.03.2025 12:38, Roger Pau Monné wrote:
> >>> On Thu, Mar 27, 2025 at 12:20:47PM +0100, Jan Beulich wrote:
> >>>> Unlike stated in the offending commit's description,
> >>>> load_system_tables() wasn't the only thing left to retain from the
> >>>> earlier restore_rest_processor_state().
> >>>>
> >>>> While there also do Misra-related tidying for the function itself: The
> >>>> function being used from assembly only means it doesn't need to have a
> >>>> declaration, but wants to be asmlinkage.
> >>>
> >>> I wonder, maybe the intention was for the MTRR restoring on the BSP to
> >>> also be done by the mtrr_aps_sync_end() call in enter_state()?
> >>>
> >>> AFAICT that will set the MTRRs uniformly on all CPUs, by calling
> >>> mtrr_set_all() just like mtrr_bp_restore(), but later in the restore
> >>> process.
> >>
> >> Hmm, yes, that's possible. The comment in set_mtrr() is somewhat misleading
> >> then, though, as for the BP the writing then isn't just "okay" but necessary.
> >> Question is whether doing this so much later is actually good enough.
> > 
> > Hm, no idea really.  We do the device restore ahead of the MTRR
> > restore, so I wonder whether we could have issues by using unexpected
> > effective cache attributes for device memory accesses as a result of
> > MTRRs not being initialized?
> 
> That's just one of the possible problems. The father the MTRRs we run with
> diverged from what firmware puts in place, the bigger the possible trouble.
> I think the restoring better is done as being switched to here again. The
> absence of any discussion of MTRRs in that earlier change leaves me pretty
> certain that the behavioral change there wasn't intended. Andrew is usually
> pretty good at spelling out all intended effects.

No objection, however for the BSP we now end up restoring the MTRRs
twice, as we will also do it in mtrr_aps_sync_end().

Might be worth to mention in the commit message that the MTRR state
was restored in mtrr_aps_sync_end() for the BSP also, but that it
might be too late.

Possibly with that somehow noted in the commit message:

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks, Roger.